Well, once more the MC51 is in bits in an attempt to fix the strange, high fps one minute low the next. My thoughts turned to the sector clip.
Essentially, the sector gear - so named because it only has teeth on one sector of the circumference of gear - is the one that pulls back the piston and lets it go when said sector runs out of teeth. It also serves two other functions one of which is to pull back the tappet plate which, in turn, pulls back the nozzle thereby letting a bb move up from the magazine. After a point, the tappet plate slides off the pin that is pulling it back and the tappet plate with attached nozzle move forward shoving the bb into the barrel and, hopefully, sealing it. A fraction of second later the piston also slams forward pushing the bb out the barrel - preferably at a reasonable pace.
Still with me? Good.
However, the thoughts behind a sector clip is that instead of a pin we have a small rounded triangular(ish) bit of plastic or metal that attaches to the pin and expands the pins influence to either side. Basically, the tappet plate now starts to be pulled back earlier and moves forward later thereby giving a bb more time to get from the magazine into a position in front of the nozzle. Fine in theory. I find that while the tappet plate moving backwards earlier is a good thing I'm not convinced that moving forward later is a benefit. In fact, I'd say it is distinctly not a great idea.
As far as I can see, the problem lies with the tappet plate not returning the nozzle to its position quickly enough. In other words, the delay to allow the bb time to get from the magazine to a position in front of the nozzle means that the piston can complete it's cycle before the tappet plate is fully forward resulting in a poor air seal and significant drop in performance. Two real options here; beef up the tappet plate spring or modify the sector clip. Removing it is also an option but you do lose the benefit of the nozzle moving back early.
With this in mind, I had a look at the sector clip on the MC51. Unlike most sector gears there is no tappet pin as such - the symmetrical sector clip and pin are two pieces of metal fixed together. My solution was basically to remove some material from the side that delays the return. As it transpires I took most of that side off leaving the clip somewhat lopsided; while the nozzle retracts earlier it returns with only minimal delay. After polishing it smooth I refitted it.
Worked like charm.
